1. Field of the Invention
The present invention relates to a device for redundant speed monitoring of a synchronous motor which is sine-commutated without the use of sensors and which has a permanently magnetically excited rotor, the speed monitoring device having a first control unit, a drive output stage, and a plurality of drive phases.
2. Description of the Prior Art
Synchronous motors which are sine-commutated without the use of sensors and which have a permanently magnetically excited rotor, have been successfully used in many applications. One of the advantages of such motor drives over other systems, e.g., block-commutated drives, consists in the very homogeneous curve of the torque generated by the rotating field. Therefore, drives of this type generate substantially fewer vibrations, which is desirable or even required in many applications.
These drives are often used in machines that must meet safety regulations. One of these safety regulations is that the speed must be monitored, and a maximum speed may not be exceeded. Another regulation is that the monitoring must be protected against a malfunction of the monitoring unit. It is conventional in the prior art to carry out speed monitoring by a specially installed speed sensor. This is because in the case of powering with a sinusoidal current, in contrast to block commutation, all phases must be supplied with power at all times. Whereas it is possible in block commutation to avoid the speed sensor (for example, a Hall sensor or a Hall IC) by measuring the voltage induced in the respective unpowered coil, this solution is not available in the case of powering with the sinusoidal current.
The drawback of these sensors consists in that, first of all, they constitute an additional structural component part in the area of the drive. This means, among other things, that cables and plug-in connections must be provided in order to evaluate the sensor. The second problem is that such sensors are prone to malfunction, so that more than one of them must be provided in order to implement truly reliable monitoring. In addition, the sensor components are sensitive to chemical influences and to electromagnetic or radioactive interference.
Therefore, it is the object of the invention to provide a device for redundant speed monitoring of a sensor-free sine-commutated synchronous motor in which the above-mentioned drawbacks are eliminated.